138 Morphological Effects of Like Mechanical Conditions. [March, 
by the inferior wall of the “dorsal tube,” wv in the cut. The 
extremital portions of the 
axes in both, manifest more 
or less distinctly the action 
> of flexion in preserving the 
axial elements separate, 
since, in the necks and tails 4 
of both, there still remains ; 
more or less of mobility. In 
A, transverse section through the “ dorsal ; 
tube ” A lege cthus so ent redu tee after both, the union of the ver- 
Burmeister. B, the sam ugh the dorsal tebræ has resulted in a sim- 
region a, a aps song terrapin eio. ae 
original. C, side view of dorsal vertebra of ilar disposition of the lat- 
re ees after Bojamus. eral foramina for the exit of 
the spinal nerves, that is, these open midway of the length of the 
vertebrz, piercing the lateral wall of the dorsal tube, and not 
passing out laterally between the bony arches of the neural canal, 
as happens in other vertebrates (see A, a, B, a and C, a, of cut). 
The reduction of the centra in the higher tortoises is, as should 
be expected, much more manifest than in the lower forms, and 
the union with the carapace, though not extensive, is manifested 
in the armadilloes by sutural union in the lumbar region. Simi- 
lar structural alterations, which are believed to be similarly due to 
alterations in the mechanical relationship of the skeletal elements, 
are to be observed in the sacra of birds and mammals where the 
ilia have been greatly elongated so as to prevent lateral flexures 
of this portion of the column. The ribs in turtles have been 
involved in the dermal ossifications, and are therefore, as should 
be expected (B, 4 and C, 4), united to the vertebral axis by suture; 
this is not the case in the Hoplophoride, where, owing to the 
preservation of the more highly specialized mammalian respiratory 
apparatus, the vertebro-costal articulations are still preserved, with 
loss in large measure of movement in an outward direction of 
their sternal ends. The costal movement was probably from 
behind forwards, with return, since the only articulation of the 
= thoracic axial skeleton which is preserved, is that between the 
_ twelfth dorsal vertebra and first lumbar. .A priori we should 
expect the phylogenetic history of the vertebral axes of the order 
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1 A similar degeneration and anchylosis of the centra is observed in the cervical 
ebra of Dipodo. omys, as I have recently ascertained from a specimen sent me PY 
